1. Field Of The Invention
The present invention relates to luminaires which are to be used with high intensity light sources beneath relatively low ceilings such as found in schools, offices and shops. Specifically this invention relates to luminaires that are to be used with high intensity discharge lamps to prevent introduction of significant glare.
2. Description Of The Prior Art
High intensity discharge lamps have been used to light rooms although they are more commonly applied to outdoor illumination. These lamps have a very high luminous efficiency, frequently in the order of 50 to 120 lumens per watt. In turn, they are quite bright, frequently producing 3,000 to 1,000,000 foot lamberts. Interior spaces have sufaces that vary from 10 to 100 foot lamberts at normal illumination levels and the introduction of illumination within such spaces should be accomplished with a system which does not produce significant glare. Glare is considered significant if luminance ratios between areas in the room exceed 10 to 1 which therefore limits normal interior brightness to a maximum of 100 to 1000 foot lamberts. Because of the brightness of the lamps, they should not be viewed directly nor should an image of the source be seen in reflectors or lenses that are associated with them.
Most interior illumination has been accomplished with the use of incandescent or fluorescent lamps. In general, fluorescent lamps are not so bright as to produce significant glare, but as with most light sources, they are shielded and controlled by lenses and diffusers to reduce and regulate luminance. Lighting fixtures are therefore normally designed so that high luminance is eliminated or seen at only certain viewing angles where glare is not considered detrimental.
One of the more conventional approaches to illumination with high intensity light sources has been to dispose the lamp axially in a bowl-shaped reflector which is mounted in or suspended from the ceiling with the mouth facing downwardly. The use of this fixture without further modification, however, is impractical even with relatively high ceilings. Placing a refractor or lens over the mouth of the fixture reduced the direct glare, somewhat, but this approach has produced other problems. High intensity discharge lamps radiate a substantial amount of heat, and entrapping the heat within the fixture can distort the emission color of the lamp and shorten its life. Dirt which collects on the refractor or lens must be kept to a minimum with periodic maintenance or else the glare control capabilities are lost.
Other methods of controlling glare from conventional systems include special reflector designs which by their shape or combination with prismatic refractors limit light in directions which are within the normal viewing range. The mouth of the luminaire often includes cones which are blackened or otherwise reduced in brightness to limit the ability to see high luminance elements. Louvers are also used over the mouth for this purpose. Each of these elements absorbs energy which decreases the efficiency of the system.
Any attempt to incorporate high intensity discharge lamps into an indirect luminaire with no consideration for optical control could result in a unit whose appearance is somewhat similar to that illustrated by this invention. The photometric performance of such a luminaire could be similar to that shown in FIG. 3. This illustrates that such a luminaire might substantially eliminate light from below the horizontal but most of the energy would be in the 90.degree. zone above the luminaire (45.degree. on each side of the vertical). This concentration of energy would be unacceptable for most installations, since the ceiling adjacent to the luminaire would receive most of the light. To achieve acceptable uniformity of illumination across the ceiling, an optical design such as that illustrated by this invention is required.